Objective: To investigate the short-term toxicity of deltamethrin in rats. Materials and methods: Deltamethrin was dissolved in dimethyl sulphoxide and the oral LD50 was calculated to be 150 mg/kg. Then groups of rats were given repeated daily oral dose (1/10 LD50) of deltamethrin for 30 days. The animals were killed on 31st day. Activities of various enzymes, cytochrome P450 and b5 contents in liver, hepatic antioxidant status, tissue residue concentration, haemogram and pathological changes were studied. Results: Deltamethrin increased the serum aminotransaminase, alkaline phosphatase, lactate dehydrogenase activities and blood glucose level significantly. Deltamethrin decreased PCV and Hb level and the liver cytochrome P450 content significantly. Considerable amount of residues were present in different tissues. It increased malondialdehyde level, while decreased the activities of catalase, superoxide dismutase, reduced glutathione and glycogen levels in liver significantly. Mild to moderate histological alterations were observed in lungs, liver, stomach, kidney, testes and cerebellum. Conclusion: Repeated dose toxicity studies of deltamethrin at 1/10 LD50 altered the biochemical parameters; decrease cytochrome P450 content, antioxidant status, which correlated with histopathological changes with considerable amount in tissue residues.